Interferometric measuring means are often used for highly precise distance and position measurements. In a high-precision interferometric measurement the relative difference in distance between a measuring beam and a reference beam is usually measured with the beam path of an interferometric measuring means. A beam splitter of the interferometric measuring means is for dividing the light generated by a laser into a measuring beam and a reference beam. The measuring beam passes between a beam splitter of the interferometric measuring means and a measuring mirror, wherein the measuring mirror is arranged at a moveable measuring object, i.e., for example, on a positioning stage. The reference mirror is fixed with respect to the beam splitter of the interferometric measuring means. The measurement serves to determine how the phase of the light changes as the measuring object moves. The measuring accuracy is dependent, apart from the correct adjustment of the optical axes or the optics itself, on the signal processing (electronics, software) and the calibration of the overall system, also on the ambient conditions, such as the composition of the ambient air, moisture, temperature and pressure. The wavelength of the light beam is the basic measuring unit of the measurement. The relative difference in distance is therefore indicated in the unit “wavelength”. The current value of the wavelength of a light beam is a function of the refractive index of the medium passed by the light beam. The refractive index varies, for example, due to gradual variations or rapid fluctuations of the temperature, air pressure and moisture, or due to changes in the air composition. The fluctuations in the readings due to wavelength variations, at ±0.1 μm, are no longer negligible with respect to the structures to be measured in typical measurements, such as in a coordinate measuring device on wafers and masks, and are therefore no longer acceptable for the required measuring accuracy. To increase the measuring accuracy it is therefore necessary to take into account wavelength variations of the light beam by means of a continuous wavelength correction.
For high precision distance measurements, a coordinate measuring device, such as it is known from DE 198 19 492 could be operated in a climatic chamber. Herein, at least the temperature is maintained at a constant level, in some climatic chambers the moisture is also maintained at a constant level. The control accuracy of temperature and moisture has technical limits. With a reasonable expenditure of effort, it is impossible to create a hermetically sealed chamber to maintain a constant air pressure, in particular since in the example of a coordinate measurement device it is necessary that the measuring objects can be exchanged simply and rapidly. Even the operation of the loading door causes rapid pressure fluctuations. The ambient conditions are defined by a climatic chamber via the feedback of temperature sensors and humidity sensors, and via over-pressure adjustments. “Over-pressure” refers to a positive pressure differential of a few Pascal with respect to the ambient pressure of the climatic chamber. Fluctuations of the air pressure within the chamber are caused by the measuring structure itself and are therefore not entirely avoidable. Further tolerances for the chamber are: with respect to the temperature 1/100 degree (in time), 2/100 degree spatially and with respect to the humidity ±1%. The measuring signals are detected in the X and Y directions with a nominal detector accuracy of Δλ/2048˜0.3 nm (λ=laser wavelength=632.9 nm). As a result the interferometer wavelength must be continuously detected in a separate measurement. This can be done by measuring a wavelength calibration distance with a constant length, a “wavelength tracker”, or by measuring the influential factors, such as temperature, air moisture etc., while continuously calculating the current wavelength. One example of such an interferometric correction means is the commercially available laser interferometer HP 10702 of Agilent (formerly Hewlett Packard).
The light generated by a laser is divided into two partial beams by a beam splitter of the interferometric correction means. The two partial beams pass between the beam splitter of the interferometric correction means and a reference mirror fixed with respect to the beam splitter of the interferometric correction means. The two partial beams, also referred to as reference beams in the following, have optical paths of different lengths. Thus for a coordinate measuring device a wavelength tracker is usually provided which is arranged at a site on the coordinate measuring device that seems to be suitable for detecting changes in ambient conditions and where basically there is structural space available for it. It is necessary, however, to always point out possibilities for increasing the measuring accuracy of a coordinate measuring device or a measuring system. This can also be achieved by increasing the accuracy when determining changes in ambient conditions.
In a coordinate measuring device, the object to be measured is positioned or moved with the aid of a positioning stage relative to the detection optics in order to optically detect each area to be imaged by the optics of the usually planar surface of the object to be measured. Herein the surface of the usually square or circular shape of the object has a size of between about 10 cm×10 cm and about 30 cm×30 cm, or a diameter of about 10 cm to about 30 cm. In order to inspect the whole surface of the object with the aid of the coordinate measuring device, it should be possible to position the object with the positioning stage on a traversing path essentially corresponding to the size or the dimensions of the surface of the object. This is why the maximum traversing path of the moveable positioning stage is usually a function of the size of the surface of the object normally to be inspected by the coordinate measuring device. Similar considerations generally apply to measuring assemblies in which the most stringent requirements apply to the measuring accuracy.